QR allows noninvasive, short-range detection of materials containing nitrogen, including many explosives. Unlike other technologies, QR detection systems can discriminate among different types of explosives and distinguish them from benign nitrogen compounds, because the QR response from each nitrogen compound has a unique spectral signature.
There is a great current need to detect explosives concealed within containers, such as luggage, mail, improvised explosive devices, and minimal metal landmines. At the present time, x-ray detection is the primary technology used at aviation security checkpoints. X-ray detection systems reveal the presence and shape of objects that absorb energy form the x-ray beam, but cannot distinguish between benign material and explosive devices. Current QR detection systems are hindered by problems such as inadequate sensitivity, limited operating temperature range, and electrical interference from both internal and external RF sources.
The signal-to-noise ratio (SNR) of a nuclear resonance measurement using QR is proportional to the square-root of the Quality-factor (Q-factor) of the probe. Q-factors are approximately 100 for conventional (non superconducting) metal coils, which severely limits the SNR.
The frequency of the QR response from a particular nitrogen compound is temperature dependent, and existing commercial QR detection systems require that the searched objects be held within small temperature range. Further, the small QR response is easily masked by RF sources, such as AM broadcast stations and engine ignition noises, that are external to the detection system.
In conventional detectors, excitation of a QR response requires the application of a pulsed RF magnetic field within the search volume. The applied RF pulse may excite spurious responses from materials within the search volume that can obscure the QR response, leading to an unacceptably large false alarm rate. Examples of internal noise sources include the decaying magnetic field generated by currents induced within conductive materials located within the search volume, as well as piezoelectric responses from materials within the search volume. In order to excite a QR response, the amplitude of the RF field must be larger than several Gauss over the search region. This level of amplitude requires the use of a costly power amplifier and excitation probe, and exceeds the allowable limits for human exposure.
In contrast, conventional CW detection systems excite a QR response by applying a steady-state RF magnetic field whose amplitude is orders of magnitude smaller than that of pulse excitation systems. As the excitation filed is monochromatic, it must be swept over the spectral region that most likely to contains the QR transition. As a result, conventional CW detection systems achieve a useful SNR at the expense of an impractically large detection time for security applications. The minimum detection time is determined by the bandwidth of a lock-in amplifier, which is often less than one Hertz, resulting in detection times spanning tens of minutes. For this reason detection of explosives using CW QR methods has not been pursued.